In PET imaging, or positron emission tomography, a radiopharmaceutical agent is administered, via injection, inhalation and/or ingestion, to a patient. The physical and bio-molecular properties of the agent then concentrate at specific locations in the human body. The actual spatial distribution, intensity of the point and/or region of accumulation, as well as the kinetics of the process from administration and capture to eventual elimination, all have clinical significance. During this process, the positron emitter attached to the radiopharmaceutical agent emits positrons according to the physical properties of the isotope, such as half-life, branching ratio, etc. Each positron interacts with an electron of the object, is annihilated and produces two gamma rays at 511 keV (electron-positron annihilation event), which travel at substantially 180 degrees apart. The two gamma rays then cause a scintillation event at a scintillation crystal of the PET detector, which detects the gamma rays thereby. By detecting these two gamma rays, and drawing a line between their locations or “line-of-response,” the likely location of the original annihilation is determined. While this process only identifies one line of possible interaction, accumulating a large number of these lines, and through a tomographic reconstruction process, the original distribution is estimated with useful accuracy.
Each PET detector may also include, in addition to the scintillation crystal, a photomultiplier tube (PMT) or a silicon photomultiplier (SiPM) that is optically coupled to the scintillator. Each PMT/SiPM has a property denoted as “photosensor gain”. In the context of a PMT this gain is defined as the total number of electrons that are collected at the anode of the PMT as a result of an event.
In order to ensure proper operation of the PET system, the photosensor gain for each PMT/SiPM must be consistently re-measured and addressed due the drifting nature of the photosensor gain. This calibration process is typically performed by measuring photo-current induced by an external light source that is different from the scintillator. This process is difficult when the PMT/SiPM is assembled as a part of the detector, and especially when direct measurement of the photosensor gain is unfeasible.
The “background” description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description which may not otherwise qualify as prior art at the time of filing, are neither expressly or impliedly admitted as prior art against the present embodiments.